Mosaic Co., the world’s largest phosphate company, has spent two decades lining up a new mine in DeSoto County as part of a broader effort to move its operations south.

But DeSoto County commissioners last week slammed the door in the company’s face, voting 4-1 against rezoning 18,000 acres from agricultural to mining.

A major concern: The impact of mining on one of the state’s most pristine waterways, Horse Creek. The creek is a major tributary of the Peace River, which supplies water for three counties.

The commissioners voted down Mosaic’s zoning request Wednesday night after two days of impassioned testimony from hundreds of people opposed to the mine. The rejection — after their own planning director recommended approval — came as a surprise to many, including longtime mining foe Dennis Mader.

"After 16 years of activism I have never seen a victory that decisive," said Mader, president of People Protecting the Peace River. "Hopefully this will turn the tide on them."

A Mosaic spokeswoman said the company was "disappointed" with the decision, and hinted at possible legal action.

"It is clear that our rezoning application met every requirement of law and we will now begin reviewing our options to ensure our rights are protected," said company spokeswoman Jackie Barron.

Although phosphate was first discovered in what is now DeSoto County, no one has mined there in a century because the richest deposits were farther north, in Polk County. Phosphate miners have spent decades digging up millions of tons of the fertilizer ingredient there, processing it and shipping it out of the Port of Tampa.

But now that those mines are playing out, the company has spent the past 20 years working toward opening or expanding mines in Manatee, Hardee and DeSoto counties. The effort has required a lot of long-range planning by the company. For instance, it applied for a state permit for the DeSoto mine in 2014, winning approval from the Department of Environmental Protection in 2017.

The U.S. Army Corps of Engineers, seeing so many new phosphate mines looming on the horizon, launched a wide-ranging study of their environmental impact. The study found they would destroy nearly 10,000 acres of wetlands and 50 miles of streams, causing a "significant impact."

But the two-year study — prepared for the federal agency by a consultant paid by the phosphate industry — contended the miners would do such a good job of making up for the damage eventually that it wouldn’t matter.

Mosaic is so committed to the expansion of its footprint into new territory that it recently announced it’s moving its corporate headquarters from Minnesota to Florida.

As Mosaic has tried to move into the future, though, it has had to deal repeatedly with questions about the industry’s past damage to the environment. That includes a 1994 accident involving its predecessor, IMC-Agrico, that sent 500 million gallons of slimy, gray water rolling toward the Alafia River, flooding homes, killing livestock, mucking up ponds and contaminating private wells.

More recently, a sinkhole opened in 2016 under a phosphogypsum stack at Mosaic’s Mulberry plant near the Hillsborough-Polk county line, sucking 215 million gallons of contaminated water into the aquifer.

In the latter case, the company failed to disclose the disaster to the surrounding community for three weeks, which angered residents once the news got out. Sealing the sinkhole took two years, $84 million and 20,000 cubic yards of grout, a thick mixture of water, cement, and sand which hardens over time.

At the DeSoto County hearing, a hastily organized contingent of charter boat captains from downstream of the proposed mine showed up in black T-shirts that said, "Phosphate Kills: A 1971 spill killed 90% of the fish in the 108-mile long Peace River."

Opponents of Mosaic’s DeSoto mine plans were particularly concerned about its impact on their drinking water. The mine would be adjacent to Horse Creek, which provides 15 percent of the freshwater flow to the Peace River.

The Peace River-Manasota Regional Water Supply Authority provides more than 28 million gallons of water a day to customers in Sarasota, Charlotte and DeSoto counties.

The creek is so pure that a water authority official once observed that the Peace River’s purity is "barely acceptable for water supply purposes … until the waters of Horse Creek enter it."

When commissioners asked questions of Mosaic, they didn’t like the answers, Mader said. One question they asked: How much of the mine’s process water would be dumped into the creek? The answer, according to Mader: anywhere from 1 million to 30 million gallons a day. And while Mosaic would check the quality of the discharge once a week, it would be tested by an outside party no more than once a year.

Last year, Manatee County commissioners approved a 3,600-acre expansion to a Mosaic mine that sits right next to the Lake Manatee watershed. Mosaic went back to the Manatee commissioners Thursday to get approval for its operating permit.

Manatee County was the last county to try to deny Mosaic what it wanted.

In 2008, Manatee commissioners voted 4-3 to reject Mosaic’s application to expand its Four Corners Mine to a 2,048-acre site commonly called the Altman Tract. Mosaic filed a $617.8 million suit against the county. A year later — after an election that changed the composition of the commission — the county reversed its denial.

Mosaic filed that lawsuit under the state’s Bert Harris Act. The law, passed in 1995, says local governments cannot place such an "undue burden" on what a landowner can do with his or her land that it prevents them from using it for a profit — regardless of the impact on neighbors or those who live and work downstream.

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